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There are many factors when calculating a probability of spam.
Reverse DNS is important.
SPF is not very important but can help a tiny bit if configured correctly and can hurt a lot if configured incorrectly.

Also the subject of email is important and the content of the body as well. If your email body is very short like "hello, this is a test", this can be considered a spam.

Also, look at the return-path in your email headers. If you sending via web form, then it could actually be sent from the account under which your webserver runs, which is 'nobody' in many cases and this is also bad for spam filters.

Also, very important is the IP address of your server. If its on a subnet that is known to spam yahoo users, then you are out of luck with yahoo.

Hi Thanks very much for your response. I am now talking to my host and they say they

we can set PTR for you, with no problem (this is free). In this case, the benefit will be that you can add an SPF record to domains hosted on your server so that e-mails sent through your server will have proper SPF and reverse lookup, resulting in less e-mails marked as spam.

Hopefully, I'll be able to get through to Yahoo and hotmail when this is done.